1. Field of the Invention
This invention relates to the removal of impurities from a fluid stream. Particularly, this invention relates to the removal of solid substances from a gas stream. More particularly, this invention is related to the removal of wax from a gas stream.
2. Description of the Related Art
Various techniques have been developed for removal of undesirable substances from a gas stream. As illustrated in FIG. 1, one approach uses two wax traps in series to remove the wax that solidifies from the gas stream. The traps include a bed of solid, extended surface, fluid contact phase change devices, typically Rashing rings, Intalox saddles, Pall rings, Berl saddles, Lessing rings, and other commercially available devices. These phase change devices are typically poured into a wire basket that is then inserted within a pressure vessel. The pressure vessel is then cooled with chilled water re-circulating through a water jacket. Typically, the wax-containing feed stock piping, any interconnecting piping, and the pressure vessels themselves were unheated and un-insulated.
As shown in FIG. 1, the fact that the wax-containing feed stock pathway is unheated and un-insulated allows most of the wax (represented by the arrows) to precipitate on the cooler surfaces of the piping and the traps. Despite this unwanted inner surface precipitation, and even after having passed through the zone containing the phase change devices, a significant amount of wax-containing residue breaks through the system. Typically, no provisions are made to cycle the traps, as they are usually not arranged in redundant, parallel trains. Likewise, typically no provisions are made to clean these traps in-situ, heat/insulate the piping surfaces, improve wax removal efficiency, or prevent solids deposition on vessel or piping internals.
The above systems experience three main problems. The traps, including phase change devices and basket, need to be disassembled and cleaned often, in order to remove the solids which have precipitated on them. In many cases these cleanings are required daily. The entire wax remediation unit must also be disassembled and cleaned weekly to remove precipitated solids from the internals of the piping and trap vessels. Lastly, the traps themselves are reasonably inefficient, because they allow enough waxy solids downstream to cause problems with downstream components, such as vacuum pumps or other gas handling equipment.
Another approach as illustrated in FIG. 2, removes gaseous VOC's from a process waste stream via a cryo-condensation based process. Typically, the VOC-laden stream is fed to two, or more, heat exchangers, typically in parallel, to cool the process gas stream using a cryogenic fluid. These systems are designed to condense the various VOC's, and not to freeze them onto a target surface. Inevitably, some undesired freezing will occur. Thus, while one heat exchanger is being used to cool the VOC-laden stream, heat may optionally be used to melt off any water ice or VOC's from the cold surfaces of other heat exchangers. The resultant condensed liquid VOC material is then typically drained away from the heat exchanger via gravity and held in a storage vessel to be pumped out. Remaining clean waste gas exits the system. Examples of such systems are the SOLVAL VOC Removal and Recovery System marketed by Air Liquide America, LP, and the Kryoclean VOC Control System marketed by BOC.
A VOC is defined by the Federal Government as a photochemically reactive chemical that contains carbon. 40 C.F.R. 51.100 (s). This is a broad definition, and there is no common agreement among the various states as to what constitutes the complete list of VOC's. It is generally agreed that VOC's tend to be fairly light molecules, generally ranging from around propane (MW 44.1) to around C15 Alkenes (MW 210.4). There are some VOC's with higher molecular weights, such as Branched C18 Alkanes (MW 254.5), but most tend to be fairly light.
Another generally agreed common trait is that most VOC's tend to be in liquid phase at ordinary ambient temperatures. It is well known to those of ordinary skill in the art, that for organic compounds in the same class, both the boiling point and the freezing point increase with carbon number, and thus molecular weight. Variances in class (i.e. aromatic, napthene, isoparrafin, etc.) also effect boiling point and freezing point.
The present invention is useful for removing molecules that tend to be heavier than most VOC's, in particular hydrocarbon molecules with carbon numbers of C12 and larger. Hydrocarbon molecules with carbon numbers of C12 and higher, and more particularly those with carbon numbers of C15 and higher, tend to be solid at ordinary ambient temperatures. These higher carbon number molecules also tend to be waxy in nature, and herein the term ‘wax’ or ‘waxy’ will be used synonymously with these higher carbon number molecules.
The present invention uses a freezing mechanism to capture and collect such solid pollutants, and subsequent heating to greater than ambient temperature is used to melt off the accumulated wax. In addition, the front end piping and wax storage capacity may be heated and insulated in the invention to prevent solids precipitation in unwanted areas and to facilitate removal of the previously melted wax via draining.
Other types of cryogenic or non-cryogenic (Thermal Oxidizer, Carbon Adsorption, Water spray towers, etc.) systems for VOC abatement are also commercially available from industrial companies. Many of these systems use catalysts or solvents to aid in the removal of the hydrocarbon impurities. The present invention introduces no new substances that may potentially act to contaminate the fluid stream.
The prior art systems are good at condensing VOC's, and removing them from the stream in liquid form. However, these systems are not used for removal of substances existing as solids at room temperature. Therefore, a need exists within the industry for a system that can be used to remove higher molecular weight compounds, particularly those that are solid at ordinary ambient temperatures, from a fluid stream. In particular, a need exists within the industry for a system that can be used to remove vaporized or suspended higher carbon number components that tend to be solid at ordinary ambient temperatures, from a gas phase stream.